I aged in my 20s. I also aged in my 30s and 40s as well.
That bothered me, too - until I realized that it is actually less ambiguous than writing "a man in his 20s," since that doesn't specify age, and thus could instead be referring to weight ("between 20 and 30 kgs?!").
Still, the convention is that it refers to age, as "measured in your Earth years." ("Journey to Babel" reference)
Regards,